Field of the Invention
The present invention relates to a photoelectric conversion device and an imaging system.
Description of the Related Art
In general, a photoelectric conversion device is required to have a high sensitivity and a wide signal output range (dynamic range) with respect to the amount of incident light.
Japanese Patent Application Laid-Open No. 2006-041866 discloses a photoelectric conversion device in which a photoelectric conversion unit and a pair of transfer elements are provided in each of a plurality of unit pixels arranged in an imaging area, and a charge holding unit is shared between two unit pixels adjacent in a vertical direction. Then, a signal charge accumulated for a long period of time is read from one photoelectric conversion unit of the two unit pixels and a signal charge accumulated for a short period of time is read from the other into the charge holding unit at the same time to expand the signal output range with respect to the amount of incident light.
Japanese Patent Application Laid-Open No. 2012-049904 discloses a photoelectric conversion device having unit pixels in each of which a charge holding unit and a reset unit are shared between a pair of photoelectric conversion units. According to the photoelectric conversion devices disclosed in Japanese Patent Application Laid-Open No. 2006-041866 and Japanese Patent Application Laid-Open No. 2012-049904, signal charges accumulated in two photoelectric conversion units are read into one charge holding unit so that the signal amplitude with respect to the amount of incident light can be enlarged.
In the techniques of Japanese Patent Application Laid-Open No. 2006-041866 and Japanese Patent Application Laid-Open No. 2012-049904, signal charges are read into the same charge holding unit in both a case where a signal charge is read from only one photoelectric conversion unit and a case where signal charges are read from a plurality of photoelectric conversion units. Therefore, the charge holding unit has limitations placed on the design thereof to be responsive to both the case of reading a signal charge from only one photoelectric conversion unit and the case of reading signal charges from the plurality of photoelectric conversion units. For example, when the capacitance value of the charge holding unit is set large to be responsive to reading from the plurality of photoelectric conversion units, the sensitivity is deficient when a signal is read from only one photoelectric conversion unit. On the other hand, if the capacitance value of the charge holding unit is set small to obtain high sensitivity when reading a signal from only one photoelectric conversion unit, it will increase the probability of being saturated even with a small amount of light in reading from the plurality of photoelectric conversion units.